Background The diagnosis of sonographically indeterminate adnexal masses (AM) signifies a major challenge in clinical practice. Early detection and characterization have increased the need for accurate imaging evaluation before treatment. Purpose To assess the validity and reproducibility of the ADNEX MR Scoring system in the diagnosis of sonographically indeterminate AM. Study Type A prospective multicenter study. Population In all, 531 women (mean age, 44 ± 11.2 years; range, 21–79 years) with 572 sonographically indeterminate AM. Field Strength/Sequence 1.5T /precontrast T1 ‐weighted imaging (WI ) fast spin echo (FSE) (in‐phase and out‐of‐phase, with and without fat suppression); T2‐WI FSE ; diffusion‐WI single‐shot echo planner with b‐values of 0 and 1000 s/mm2; and dynamic contrast‐enhanced perfusion T1‐WI liver acquisition with volume acceleration (LAVA) . Assessment All MRI examinations were evaluated by three radiologists, and the AM were categorized into five scores based on the ADNEX MR Scoring system. Score 1: no AM; 2: benign AM; 3: probably benign AM; 4: indeterminate AM; 5: probably malignant AM. Histopathology and imaging follow‐up were used as the standard references for evaluating the validity of the ADNEX MR Scoring system for detecting ovarian malignancy. Statistical Tests Four‐fold table test, kappa statistics (κ), and receiver operating characteristic (ROC) curve. Results In all, 136 (23.8%) AM were malignant, and 436 (76.2%) were benign. Of the 350 AM classified as score 2, one (0.3%) was malignant; of the 62 AM classified as score 3, six (9.7%) were malignant; of the 73 AM classified as score 4, 43 (58.9%) were malignant; and of the 87 AM categorized as score 5, 86 (98.9%) were malignant. The best cutoff value for predicting malignant AM was score >3 with sensitivity and specificity of 92.9% and 94.9%, respectively. The interreader agreement of the ADNEX MR Scoring was very good (κ = 0.861). Data Conclusion The current study supports the high validity and reproducibility of the ADNEX MR Scoring system for the diagnosis of sonographically indeterminate AM.